The structure of N,N-dimethylethylenediammonium pentachloroantimonate(III), [(CH3)(2)NH(CH2)(2)NH3][SbCl5], NNDP, was investigated at 100 and 15 K at ambient pressure, as well as at pressures up to 4.00 GPa at room temperature in the diamond-anvil cell. The stable structure at low temperatures and low pressures consists of isolated [SbCl5](2-) anions and [(CH3)(2)NH(CH2)(2)NH3](2+) cations. The inorganic anions have a distorted square pyramidal geometry. They are arranged in linear chains parallel to the c axis. In contrast to the low-temperature studies, where no phase transition was detected, pressure induces a P2(1)/c --> P2(1)/n phase transition between 0.55 and 1.00 GPa, accompanied by a doubling of the a unit-cell parameter. This solid-solid transition results from changes in the electron configuration of the Sb-III atom and formation of the Sb-Cl bridging bonds between inorganic polyhedra to form, at similar to 1.0 GPa, isolated [Sb2Cl10](4-) units consisting of [SbCl6](3-) octahedra and [SbCl5](2-) square pyramids connected by a common corner. The intermolecular distances continuously decrease with further increase in pressure, and at similar to 3.1 GPa, zigzag [{SbCl5}(n)](2n-) chains containing corner-sharing [SbCl6](3-) octahedra are formed. The unit-cell volume of NNDP decreases by 18.15% between room pressure and 4.00 GPa. The linear distortions of the [SbCl5](2-) and [SbCl6](3-) polyhedra decrease with increasing pressure and decreasing temperature and indicate a reduction in the stereochemical activity of the lone electron pair on the Sb-III atom.